Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/0623—Sulfides, selenides or tellurides
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
  • G03G5/00—Recording members for original recording by exposure, e.g. to light, to heat, to electrons; Manufacture thereof; Selection of materials therefor
  • G03G5/02—Charge-receiving layers
  • G03G5/04—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor
  • G03G5/08—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor characterised by the photoconductive material being inorganic
  • G03G5/082—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor characterised by the photoconductive material being inorganic and not being incorporated in a bonding material, e.g. vacuum deposited
  • G03G5/08207—Selenium-based

Definitions

• the present invention relates in general to a process for vacuum depositing a selenium alloy layer onto a substrate for electrophotographic imaging members.
• electrophotographic imaging members for xerography, i.e. photoreceptors, can be used in the electrophotographic imaging process.
• electrophotographic imaging members may include inorganic materials, organic materials, and mixtures thereof.
• Electrophotographic imaging members may comprise contiguous layers in which at least one of the layers performs a charge generation function and another layer forms a charge carrier transport function or may comprise a single layer which performs both the generation and transport functions. These electrophotographic imaging members may be coated with a protective overcoating to improve wear.
• Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed and to improve color copyability. These devices are typically based on alloys of selenium with tellurium and/or arsenic.
• the selenium electrophotographic imaging members may be fabricated as single layer devices comprising a selenium-tellurium, selenium-arsenic or selenium-tellurium-arsenic alloy layer which performs both charge generation and charge transport functions.
• the selenium electrophotographic imaging members may also comprise multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium alloy generator layer.
• a common technique for manufacturing photoreceptor plates involves vacuum deposition of a selenium alloy to form an electrophotographic imaging layer on a substrate.
• Tellurium is incorporated as an additive for the purpose of enhancing the spectral sensitivity of the photoconductor.
• Arsenic is incorporated as an additive for the purpose of improving wear characteristics, passivating against crystallization and improving electricals.
• the tellurium addition is incorporated as a thin selenium-tellurium alloy layer deposited over a selenium alloy base layer in order to achieve the benefits of the photogeneration characteristics of SeTe with the beneficial transport characteristics of SeAs alloys.
• Fractionation of the tellurium and/or arsenic composition during evaporation results in a concentration gradient in the deposited selenium alloy layer during vacuum evaporation.
• fractionation is used to describe inhomogeneities in the stoichiometry of vacuum deposited alloy thin films. Fractionation occurs as a result of differences in the partial vapor pressure of the molecular species present over the solid and liquid phases of binary, ternary and other multicomponent alloys. Alloy fractionation is a generic problem with chalcogenide alloys.
• a key element in the fabrication of doped photoreceptors is the control of fractionation of alloy components such as tellurium and/or arsenic during the evaporation of selenium alloy layers.
• Tellurium and/or arsenic fractionation control is particularly important because the local tellurium and/or arsenic concentration at the extreme top surface of the structure, denoted as top surface tellurium (TST) or top surface arsenic (TSA), directly affects xerographic sensitivity, charge acceptance, dark discharge, copy quality, photoreceptor wear and crystallization resistance.
• TST top surface tellurium
• TSA top surface arsenic
• tellurium enrichment at the top surface due to fractionation can cause undue sensitivity enhancement, poor charge acceptance and enhancement of dark discharge.
• tellurium enrichment at the upper surface of the tellurium alloy layer can result in similar undue sensitivity enhancement, poor charge acceptance, and enhancement of dark discharge.
• Once method of preparing selenium alloys for evaporation is to grind selenium alloy shot (beads) and compress the ground material into pellet agglomerates, typically 150-300 mg. in weight and having an average diameter of about 6 millimeters (6,000 micrometers).
• the pellets are evaporated from crucibles in a vacuum coater using a time/temperature crucible designed to minimize the fractionation of the alloy during evaporation.
• One shortcoming of a vacuum deposited selenium-tellurium alloy layer in a photoreceptor structure is the propensity of the selenium-tellurium alloy at the surface of the layer to crystallize under thermal exposure in machine service.
• the addition of up to about 5 percent arsenic to the selenium-tellurium alloy was found beneficial without impairment of xerographic performance. It was found that the addition of arsenic to the composition employed to prepare the pellet, impaired the capability of the process to control tellurium fractionation.
• Selenium-tellurium-arsenic pellets produced by the pelletizing process exhibited a wider variability of top surface tellurium concentrations compared to selenium-tellurium pellets.
• top surface tellurium concentrations were manifested by a correspondingly wider distribution of photoreceptor sensitivity values than the top surface tellurium concentration variations in the selenium-tellurium alloy pellets.
• photoreceptor sensitivity values were manifested by a correspondingly wider distribution of photoreceptor sensitivity values than the top surface tellurium concentration variations in the selenium-tellurium alloy pellets.
• up to 50 percent of the selenium-tellurium-arsenic pellets were rejected for forming high top surface tellurium concentrations which caused excessive sensitivity in the final photoreceptor.
• a sufficiently high concentration of top surface arsenic causes reticulation of the surface of the deposited alloy layer. This occurs as the deposited surface cools down and the differential thermal contraction through the thickness of the layer causes the surface to wrinkle.
• the deposited layer also exhibits electrical instability with excessive dark decay under certain conditions.
• the photoreceptor comprises a single layer Se-As alloy, about 1 to about 2.5 percent by weight arsenic, based on the weight of the entire layer, at the surface of an alloy layer provides protection against surface crystallization.
• concentration of arsenic is greater than about 2.5 percent by weight, reticulation or electrical instability risks become higher. However, the shift in photosensitivity is not large.
• shutters have been used over crucibles to control fractionation. These shutters are closed near the end of the evaporation cycle.
• tellurium or arsenic rich material arising from the crucible deposits on the shutter rather than on the photoreceptor.
• installation of shutters is complex, difficult and expensive. Further, after one or more coating runs, it is necessary to clean the surface of the shutters and the resulting debris can cause defects to occur in subsequently formed photoreceptor layers.
• a significant problem encountered in the fabrication of selenium alloy photoreceptors is the fractionation or preferential evaporation of a species such that the resulting film composition does not replicate the original composition.
• the deposited film or layer does not have a uniform composition extending from one surface to the other.
• tellurium is the dopant
• the tellurium concentration is unduly high at the top surface and approaches zero at the bottom of the vacuum deposited layer.
• the selenium-arsenic alloy may be at least partially crystallized by placing the selenium alloy in shot form in a crucible in a vacuum coater and heating to between about 93° C. (200° F.) and about 177° C. (350° F.) for between about 20 minutes and about one hour to increase crystallinity and avoid reticulation.
• the selenium-arsenic alloy material in shot form is heated until from about 2 percent to about 90 percent by weight of the selenium in the alloy is crystallized.
• the selenium-arsenic alloy material shot may be crystallized completely prior to vacuum deposition to ensure that a uniform starting point is employed.
• halogen doped selenium-arsenic alloy shot contained about 0.35 percent by weight arsenic, about 11.5 parts per million by weight chlorine, and the remainder selenium, based on the total weight of the alloy was heat aged at 121° C. (250° F.) for 1 hour in crucibles in a vacuum coater to crystallize the selenium in the alloy. After crystallization, the selenium alloy was evaporated from chrome coated stainless steel crucibles at an evaporation temperature of between about 204° C. (400° F.) and about 288° C. (550° F.).
• the large particles of the alloy may be beads of the alloy having an average particle size of between about 300 micrometers and about 3,000 micrometers or pellets having an average weight between about 50 mg and about 1000 mg, the pellets comprising compressed finely ground particles of the alloy having an average particle size of less than about 200 micrometers prior to compression.
• the process comprises mechanically abrading the surfaces of beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers while maintaining the substantial surface integrity of the beads to form a minor amount of dust particles of the alloy, grinding the beads and the dust particles to form finely ground particles of the alloy, and compressing the ground particles into pellets having an average weight between about 50 mg and about 1000 mg.
• mechanical abrasion of the surface of the pellets after the pelletizing step may be substituted for mechanical abrasion of the beads.
• the process includes providing beads of an alloy comprising selenium, tellurium and arsenic having an average particle size of between about 300 micrometers and about 3,000 micrometers, grinding the beads to form finely ground particles of the alloy having an average particle size of less than about 200 micrometers, compressing the ground particles into pellets having an average weight between about 50 mg and about 1000 mg, and mechanically abrading the surface of the pellets to form alloy dust particles while maintaining the substantial surface integrity of the pellets.
• the process may include both the steps of mechanically abrading the surface of the beads and mechanically abrading the surface of the pellets.
• the selenium-tellurium-arsenic alloy in the pellets may then be vacuum deposited to form a photoconductive layer of an electrophotographic imaging member which comprises a substrate and, optionally, one or more other layers.
• the photoreceptor is prepared by heating a mixture of selenium-arsenic alloys in a vacuum in a step-wise manner such that the alloys are sequentially deposited on the substrate to form a photoconductive film with an increasing concentration of arsenic from the substrate interface to the top surface of the photoreceptor.
• a mixture of 3 selenium-arsenic alloys are maintained at an intermediate temperature in the range of from about 100+ to 130° C. for a period of time sufficient by dry the mixture.
• the alloy may also contain a halogen.
• the drying step temperature was attained in about 2 minutes and maintained for a period of approximately 3 minutes.
• a multilayered electrophotographic imaging member in which one of the layers may comprise a selenium-tellurium-arsenic alloy.
• the alloy can be prepared by grinding selenium-tellurium-arsenic alloy beads, with or without halogen doping, preparaing pellets having an average diameter of about 6 mm from the ground material, and evaporating the pellets in crucibles in a vacuum coater.
• Additives mentioned include Te, As, Sb, Bi, Fe, Tl, S, l, F, Cl, Br, B, Ge, PbSe, CuO, Cd, Pb, BiCl 3 , SbS 3 , Bi 2 , S 3 , Zn, CdS2, SeS and the like.
• tablets having a thickness of 2mm and a diameter of 6 mm were sintered and degassed at about 210° C. for about 18 minutes.
• single crystal particles are heat treated by using a heat treatment cycle during the initial stages of which incipient melting occurs within the particles being treated. During a subsequent step in heat treatment process substantial diffusion occurs in the particle.
• single crystal articles which have previously undergone incipient melting during a heat treatment process are prepared by a heat treatment process.
• a single crystal composition of various elements including chromium and nickel is treated to heating steps at various temperatures.
• a stepped treatment cycle is employed in which an alloy is heated to a temperature below about 25° F. of an incipient melting temperature and held below the incipient melting temperature for a period of time sufficient to achieve a substantial amount of alloy homogenization.
• Japanese Patent Publication No. J6 0172-346-A published Sept. 5, 1985 - TlSe are placed in a crucible and heated at 180°-190° C., Mg is added to the melting alloy, the temperature is raised to the 200°-220° C. and allowed to stand at this temperature to form a uniform alloy of TlMgSe.
• the alloy is used in electric field-releasing ion beam generators.
• U.S. Pat. No. 3,785,806 to Henrickson issued Jan. 15, 1974--A process is disclosed for producing arsenic doped selenium by mixing finely divided selenium with finely divided arsenic in an atomic ratio of 1:4 and thereafter heating the mixture in an inert atmosphere to obtain a master alloy.
• the master alloy is then mixed with molten pure selenium to attain an arsenic content of between 0.1 and 2% by weight based on the selenium.
• an electrophotographic imaging member comprising providing in a vacuum chamber at least one crucible containing particles of an alloy comprising selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, providing a substrate in the vacuum chamber, applying a partial vacuum to the vacuum chamber, and rapidly heating the crucible to a temperature between about 250° C. and 450° C. to deposit a thin continuous selenium alloy layer on the substrate.
• a plurality of selenium containing layers may be formed by providing in a vacuum chamber at least one first layer crucible containing particles of selenium or a selenium alloy, at least one second layer crucible containing particles of an alloy comprising selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, and a substrate, applying a partial vacuum to the vacuum chamber, heating the particles in the first layer crucible to deposit a thin continuous selenium or a selenium alloy first layer on the substrate, maintaining the particles in the second layer crucible at a first temperature below about 133° C.
• the temperature of the crucible is rapidly raised to between about 300° C. and about 375° C. in between about 5 minutes and about 18 minutes.
• the substrate may be opaque or substantially trasparent and may comprise numerous suitable materials having the required mechanical properties.
• the entire substrate may comprise the same material as that in the electrically conductive surface or the electrically conductive surface may merely be a coating on the substrate.
• Any suitable electrically conductive material may be employed.
• Typical electrically conductive materials include, for example, aluminum, titanium, nickel, chromium, brass, stainless steel, copper, zinc, silver, tin, and the like.
• the conductive layer may vary in thickness over substantially wide ranges depending on the desired use of the electrophotoconductive member. Accordingly, the conductive layer may generally range in thickness from about 50 Angstrom units to many centimeters. When a flexible electrophotographic imaging member is desired, the thickness may be between about 100 Angstrom units to about 750 Angstrom units.
• the substrate may be of any other conventional material including organic and inorganic materials. Typical substrate materials include insulating non-conducting materials such as various resins known for this purpose including polyesters, polycarbonates, polyamides, polyurethanes, and the like.
• the coated or uncoated substrate may be flexible or rigid and may have any number of configurations such as, for example, a plate, a cylindrical drum, a scroll, an endless flexible belt, and the like.
• the outer surface of the supporting substrate preferably comprises a metal oxide such as aluminum oxide, nickel oxide, titanium oxide, and the like.
• intermediate adhesive layers between the substrate and subsequently applied layers may be desirable to improve adhesion. If such adhesive layers are utilized, they preferably have a dry thickness between about 0.1 micrometer to about 5 micrometers.
• Typical adhesive layers include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrolidone, polycarbonate, polyurethane, polymethylmethacrylate, and the like and mixtures thereof. Since the surface of the supporting substrate may be a metal oxide layer or an adhesive layer, the expression "supporting substrate" as employed herein is intended to include a metal oxide layer with or without an adhesive layer on a metal oxide layer.
• any suitable photoconductive chalcogenide alloy including binary, tertiary, quaternary, and the like alloys may be employed to form the vacuum deposited photoconductive layer.
• Preferred alloys include alloys of selenium with tellurium, arsenic, or tellurium and arsenic with or without a halogen dopant.
• Typical photoconductive alloys of selenium include selenium-tellurium, selenium-arsenic, selenium-tellurium-arsenic, selenium-tellurium-chlorine, selenium-arsenic-chlorine, selenium-tellurium-arsenic-chlorine alloys, and the like.
• Photoconductive alloys of selenium are to be distinguished from stoichiometric compounds of selenium such as arsenic triselenide (As 2 Se 3 ). Stoihiometric compounds of selenium such as arsenic triselenide appear to present less of a fractionation problem compared to alloys of selenium such as alloys of selenium and tellurium.
• a selenium alloy is defined as an intermetallic compound of selenium with other elemental additives where the ratios of constituents are inconsistent with stiochiometric compositions.
• the photoconductive alloys of selenium may be applied to a coated or uncoated substrate alone as the only photoconductive layer or it may be used in conjunction with one or more other layers, such as a selenium or selenium alloy transport layer and/or a protective overcoat layer.
• the selenium-tellurium alloy may comprise between about 5 percent by weight and about 40 percent by weight tellurium and a halogen selected from the group consisting of up to about 70 parts per million by weight of chlorine and up to about 140 parts per million by weight of iodine all based on the total weight of the alloy with the remainder being selenium.
• the selenium-arsenic alloy may, for example, comprise between about 0.01 percent by weight and about 35 percent by weight arsenic and a halogen selected from the group consisting of up to about 200 parts per million by weight of chlorine and up to about 1000 parts per million by weight of iodine all based on the total weight of the alloy with the remainder being selenium.
• the selenium-tellurium-arsenic alloy may comprise between about 5 percent by weight and about 40 percent by weight tellurium, between about 0.1 percent by weight and about 5 percent by weight arsenic and a halogen selected from the group consisting of up to about 200 parts per million by weight of chlorine and up to about 1000 parts per million by weight of iodine all based on the total weight of the alloy with the remainder being selenium.
• a halogen selected from the group consisting of up to about 200 parts per million by weight of chlorine and up to about 1000 parts per million by weight of iodine all based on the total weight of the alloy with the remainder being selenium.
• the expressions "alloy of selenium” and "selenium alloy” are intended to include halogen doped alloys as well as alloys not doped with halogen.
• the thickness of the photoconductive selenium alloy layer is generally between about 0.1 micrometer and about 400 micrometers thick.
• Selenium-tellurium and selenium-tellurium-arsenic alloy photoconductive layers are frequently employed as a charge generation layer in combination with a charge transport layer.
• the charge transport layer is usually positioned between a supporting substrate and the charge generating selenium alloy photoconductive layer.
• a selenium-tellurium alloy may comprise from about 60 percent by weight to about 95 percent by weight selenium and from about 5 percent by weight to about 40 percent by weight tellurium based on the total weight of the alloy.
• the selenium-tellurium alloy may also comprise other components such as less than about 35 percent by weight arsenic to minimize crystallization of the selenium and less than about 1000 parts per million by weight halogen.
• the photoconductive charge generating selenium alloy layer comprises between about 5 percent by weight and about 25 percent by weight tellurium, between about 0.1 percent by weight and about 4 percent by weight arsenic, and a halogen selected from the group consisting of up to about 100 parts per million by weight of chlorine and up to about 300 parts per million by weight of iodine with the remainder being selenium.
• a halogen selected from the group consisting of up to about 100 parts per million by weight of chlorine and up to about 300 parts per million by weight of iodine with the remainder being selenium.
• Elevated levels of tellurium lead to excessive photoreceptor light sensitivity and high dark decay and correspondingly reduced levels of tellurium result in low light sensitivity and loss of copy quality. Elevated levels of arsenic in some applications, above about 4 percent by weight, can lead to high dark decay, to problems in cycling stability and to reticulation of the photoreceptor surface.
• the resistance of amorphous selenium photoreceptors to thermal crystallization and surface wear begins to degrade as the concentration of arsenic drops below about 1 percent by weight. As the chlorine content rises above about 70 parts per million by weight chlorine, the photoreceptor begins to exhibit excessive dark decay.
• any suitable selenium alloy transport layer may be utilized as a transport layer underlying a photoconductive selenium alloy charge generating layer.
• the charge transport material may, for example, comprise pure selenium, selenium-arsenic alloys, selenium-arsenic-halogen alloys, selenium-halogen and the like.
• the charge transport layer comprises a halogen doped selenium arsenic alloy. Generally, about 10 parts by weight per million to about 200 parts by weight per million of halogen is present in a halogen doped selenium charge transport layer. If a halogen doped transport layer free of arsenic is utilized, the halogen content should normally be less than about 20 parts by weight per million.
• Imaging members containing high levels of halogen in a thick halogen doped selenium charge transport layer free of arsenic are described, for example, in U.S. Pat. No. 3,635,705 to Ciuffini, U.S. Pat. No. 3,639,120 to Snelling, and Japanese Patent Publication No. J5 61 42-537 to Ricoh, published June 6, 1981.
• halogen doped selenium arsenic alloy charge transport layers comprise between about 99.5 percent by weight to about 99.9 percent by weight selenium, about 0.1 percent to about 0.5 percent by weight arsenic and between about 10 parts per million by weight to about 200 parts per million by weight of halogen, the latter halogen concentration being a nominal concentration.
• nominal halogen concentration is defined as the halogen concentration in the alloy evaporated in the crucible.
• the thickness of the charge transport layer is generally between about 15 micrometers and about 300 micrometers and preferably from about 5 micrometers to about 50 micrometers because of constraints imposed by the xerographic development system, constraints imposed by carrier transport limitations and for reasons of economics.
• halogen materials is intended to include fluorine, chlorine, bromine, and iodine. Chlorine is the preferred halogen because of the ease of handling and stability of chlorine in a vacuum deposited film (apparently due to lack of out diffusion).
• Transport layers are well known in the art. Typical transport layers are described, for example, in U.S. Pat. No. 4,609,605 to Lees et al and in U.S. Pat. No. 4,297,424 to Hewitt, the entire disclosures of these patents being incorporated herein by reference.
• an interface layer may be positioned between the transport layer and the charge generating photoconductive layer.
• the interface layer material may, for example, consist essentially of selenium and a nominal halogen concentration of about 50 parts by weight per million to about 2,000 parts by weight per million halogen material with the remainder comprising selenium. Minor additions of arsenic might be added but are relatively undesirable and may require additional halogen to compensate for this arsenic addition.
• the halogen concentration in the deposited interface layer will typically be somewhat less than that in the alloy evaporated in the crucible. In order to achieve optimal device properties, the actual halogen content in any final interface layer should normally be greater than about 35 parts by weight per million.
• Imaging members containing high levels of halogen in a thick halogen doped selenium charge transport layer free of arsenic are described, for example, in U.S. Pat. No. 3,635,705 to Ciuffini, U.S. Pat. No. 3,639,120 to Snelling, and Japanese Patent Publication No. J5 61 42-537 to Ricoh, published June 6, 1981.
• the use of interface layers is described in U.S. Pat. No. 4,554,230 to Foley et al, the entire disclosure of which is incorporated herein by reference.
• the particles employed in the process of this invention may in general be in either shot (bead) particle or pellet particle form. However the particles may also be in chunk or ingot form also if so desired.
• shot (bead) particles the components of selenium alloys are combined by melting the selenium and additives together by any suitable conventional technique. The molten selenium alloy is then shotted by any suitable method. Shotting is usually effected by quenching molten droplets of the alloy in a coolant such as water to form large particles of alloy in the form of shot or beads. Shotting processes for forming alloy beads are well known and described, for example, in U.S. Pat. No. 4,414,179 to S. Robinette, the entire disclosure of this patent being incorporated herein by reference.
• the alloy beads may have an average size of, for example, between about 300 micrometers and about 3,000 micrometers.
• Pellet particles may be prepared from shot particles by grinding shot particles into a powder and thereafter compressing the powder into relatively large pellets. Pelletizing of the amorphous shotted alloy is frequently utilized as a means of controlling fractionation.
• the alloy beads, or combination of the alloy beads and minor amount of dust particles formed if vigorous mechanical abrasion of the alloy beads is employed is thereafter rapidly ground in a conventional high speed grinder or attritor to form alloy particles having an average particle size of less than about 200 micrometers.
• a conventional high speed grinder or attritor Any suitable grinding device may be utilized to pulverize the bead particles to form the fine alloy particles having an average particle size of less than about 200 micrometers.
• Typical grinders include hammer mills, jet pulverizers, disk mills, and the like.
• grinding alloy beads to form alloy particles having an average particle size of less than about 200 micrometers can normally be accomplished in less than about 5 minutes. Longer grinding times may be employed, if desired.
• the fine alloy particles having an average particle size of less than about 200 micrometers are compressed by any suitable technique into large particles of alloy usually referred to as pellets having an average weight between about 50 mg and about 1000 mg.
• a pellet weight greater than about 50 mg is preferred for ease of handling.
• the pellet weight exceeds about 1000 mg, evaporation discontinuities are observed.
• the pellets may be of any suitable shape. Typical shapes include cylinders, spheres, cubes, tablets, and the like. Compression of the alloy particles into pellets may be accomplished with any suitable device such as, for example, a simple punch tableting press, a multi punch rotary tableting press, and the like.
• selenium alloy shot or pellet particles may be subjected to a variety of treatments to produce surface crystallization.
• the surface crystallization which is produced at low temperatures allows subsequent bulk precrystallization to occur at higher temperatures without the usual agglomeration of the alloy particles.
• Nucleation and growth of crystallites on the surface of the alloy particles can be induced by a variety of techniques including low temperature thermal treatment of material whose outer surface has been mechanically abraded, low temperature thermal treatment during exposure to ultraviolet radiation, electron beam irradiation, gamma ray irradiation, x-radiation, and exposure to solvents and chemical vapors.
• the thermal treatment of abraded alloy particles is the preferred method of including this surface crystallization.
• a suitable abrasion technique is described in U.S. Pat. No. 4,780,386, to M. Hordon et al, filed November 28, 1986, the entire disclosure of which is incorporated herein by reference.
• Abrasion can be carried out by merely tumbling the shot or pellet particles together in a suitable device such as a roll mill.
• the alloy beads may, in one embodiment, be mechanically abraded while maintaining the substantial surface integrity of the beads to form a minor amount of dust particles from the alloy beads.
• This "minor amount" of alloy dust particles generally comprises between about 3 percent by weight to about 20 percent by weight of the total weight of the alloy prior to mechanical abrasion.
• alloy dust particles are created by imparting a vigorous tumbling action to the bead particles.
• the abrasive action should be sufficient to create dust particles having an average particle size of less than about 10 micrometers while avoiding any substantial crushing of the bead particles. More specifically, substantial surface integrity (i.e. bead shape) of the beads is maintained when less than about 20 percent by weight of the alloy beads, based on the weight of the total alloy, is fractured during the period when the beads are mechanically abraded to form the alloy dust. In other words, although the surface of the beads may be pitted and nicked, the overall bead shape is substantially conserved for at least about 80 percent by weight of the alloy beads.
• the time that the alloy beads should be mechanically abraded depends upon numerous factors such as the size of the alloy bead batch, the type of device employed to impart mechanical abrasion to the beads, the amount of crystal nucleation sites desired, and the like.
• the abrasion time should be sufficient to generate significant levels of nucleation sites at the particle surface while maintaining the substantial surface integrity of the bead particles.
• the alloy dust particles adhere to the surface of the bead particles much like toner particles adhere to the surface of carrier particles in two component electrophotographic developer mixtures and are substantially uniformly compacted around the outer periphery of bead particles.
• Any suitable device may be utilized to mechanically abrade the alloy beads and form the alloy dust particles. Typical devices for mechanically abrading particles by tumbling include vaned roll blenders, vibrating tubs, conical screw mixers, V-shaped twin shell mixers, doublecone blenders, and the like.
• the abrasion step may be omitted, the grinding and pelletizing processes generating sufficient levels of nucleation sites that subsequent surface crystallization is readily achieved.
• pre-crystallization of the selenium alloy to form at least a thin, substantially continuous layer of crystalline material covering the outer surface of the selenium alloy particles may be carried out by heating at low temperatures.
• the expression "thin, substantially continuous layer of selenium crystals" is defined as a layer of crystalline material whose surface area coverage of the alloy particle exceeds about 80 percent and more preferably approaches 100 percent.
• Precrystallization of the alloy surface may be determined by any suitable technique. Typical techniques for detecting selenium crystallization include x-ray diffraction, electron diffraction and the like. Heating may be effected, for example, with any suitable device such as an oven.
• the particles ought to be heated to a temperature between about 50° C. and about 80° C. far below the softening temperature of the particles until at least a thin, substantially continuous layer of crystalline material is formed at the surface of said particles and the core of selenium alloy in the particles remains in an amorphous state.
• the softening temperature for any given alloy may be determined experimentally by conducting constant temperature runs at incrementally increased temperatures for different batches until the shot or beads in a given batch begin to agglomerate during the thermal treatment.
• Typical pre-crystallization techniques include subjecting each surface of shot or pellets to temperatures of between about 50° C. and about 80° C.
• a selenium alloy comprising between about 0.3 and 2 percent by weight arsenic, between about 5 and about 15 percent by weight tellurium and the remainder selenium.
• a thin, substantially continuous layer of crystalline material was formed at the surface of abraded alloy shot comprising about 0.5 percent As, about 12 ppm chlorine and the remainder Se by heating the shot at about 60° C. for about 3 hours. Aging of shot material over long periods of time at either room temperature or temperatures well below the softening temperature of the alloy will ultimately lead to complete crystallization of the material but would require very long process times and consequently a high volume of "in process" material.
• the abraded alloy shot comprising about 0.5 percent As, about 12 ppm chlorine and the remainder Se described above required many weeks of heating at about 60° C. to achieve complete crystallization.
• an important purpose of the low temperature precrystallization heat treatment is to create essentially complete crystallization of a thin, continuous layer at the outer surface of the alloy beads or pellets sufficient to prevent agglomeration during the subsequent final crystallization step conducted at higher temperatures.
• This selenium-arsenic alloy material shot may be crystallized completely prior to vacuum deposition to ensure that a uniform starting point is employed.
• the crystallization of shot using heat at relatively high temperatures can cause agglomeration of the shot particles which renders the shot particles difficult to handle, weight and the like during processing prior to introduction in a vacuum coater.
• the temperature of the shot or pellets is rapidly raised in a final bulk crystallization treatment to at least a second temperature which is below the softening temperature of the particles and which is at least about 20° C. higher than the initial low temperature thermal, pre-crystallization treatment and which lies between about 85° C. and about 130° C. to crystallize at least about 5 percent by weight of the amorphous core of selenium alloy in the particles. Crystallization of 100 percent of the amorphous core of selenium alloy in the particles is preferred for optimum control of fractionation.
• a typical heating temperature for the final, second crystallization step for selenium alloys containing about 0.5 percent by weight arsenic, based on the total weight of the alloy is, for example, between about 90° C. and 100° C.
• a typical heating temperature for selenium alloys containing about 10 percent by weight tellurium, based on the total weight of the alloy is, for example, between about 95° C. and 105° C.
• the pre-crystallized alloy shot described above comprising about 0.5 percent As, about 12 ppm chlorine and the remainder Se that was abraded in a Munson Abrader and thereafter heated at about 60° C.
• the temperature for about 3 hours to form a substantially continuous surface crystalline layer may be completely and rapidly crystallized by ramping the temperature to a much higher temperature, e.g. in the 90° C.-100° C. range for about 3 hours.
• the alloys beads did not fuse together during the precrystallization temperatures and rapidly become 100 percent crystallized while keeping their integrity as separate beads.
• --An acceptable temperature range for final crystallization 85° C.-130° C., is determined by the partial vapor pressure of each of the vapor species over the solid at the temperature of interest.
• the temperature for final crystallization should be such that there is no significant loss of selenium rich species i.e. species whose composition corresponds to a selenium level considerably higher than that represented by the nominal composition within the starting alloy.
• the high temperature final crystallization treatment is generally conducted for about 10 hours or less depending upon the degree of crystallization desired. Generally, a high temperature treatment of between about 2 hours to about 10 hours may be employed for temperatures between about 85° C. and and about 130° C.
• Shot (beads) or pellets that are 100 percent crystallized are preferred for optimum control of fractionation. Where higher manufacturing throughput is desired, some reduction of the degree of crystallization can be tolerated. For example, the degree of crystallization may, if desired, be reduced to about 5 percent for arsenic or tellurium.
• the degree of crystallization of selenium alloy shot, beads or pellets can be readily determined by X-Ray diffraction spectra. As indicated above, the pre-crystallization treatment process prevents agglomeration and allows the selenium alloy particles to readily be processed, weighed, distributed evenly within the coater crucibles and the like prior to the final heating step to evaporate the alloy during vacuum deposition. However, if desired, the selenium alloy employed in the vacuum deposition process of this invention may be completely amorphous.
• pre-crystallization and final crystallization treatments used may be effected well in advance of vacuum deposition of the alloy onto a substrate.
• intermediate heating steps may be employed between the pre-crystallization and final crystallization treatments. However, such intermediate heating steps are generally unnecessary.
• the crystallized selenium alloy shot or pellets may be mixed with amoprhous selenium alloy shot or pellets to form the crucible load for vacuum evaporation onto a substrate.
• the coating process of this invention varies depending upon the different selenium alloy materials utilized. Moreover, where the selenium alloy deposited is the only photoconductive layer in the final photoreceptor, the selenium alloy may be vacuum deposited in a conventional manner except that the temperature profile used rapidly ramps from a low "hold" temperature of less than about 130° C. to a higher final evaporation temperature, the final evaporation preferably being conducted as quickly as possible without splattering. Splattering causes surface defects. Steep temperature ramping prevents selenium rich species from coming off first from the alloy which, in turn, minimizes fractionation. The ramp profile depends upon whether the selenium alloy contains Te, As, or As and Te.
• the final evaporation is preferably conducted at the highest possible temperature without splattering.
• Typical temperature ranges for ramp heating are from an initial temperature of 20° C. to final temperature of 385° C. for alloys of Se-Te; an initial temperature of about 20° C. to a final temperature of about 450° C. for alloys of Se-As; and an initial temperature of about 20° C. to a final temperature of about 385° C. for alloys of Se-As-Te.
• the final temperature may range from about 300° C. to about 450° C. for Se-Te alloys, for about 250° C. to about 450° C. for Se-As alloys and from about 300° C. to about 450° C. for Se-Te-As alloys.
• the first layer of multiple layered photoreceptors may be deposited by any suitable conventional technique, such as vacuum evaporation.
• a transport layer comprising a halogen doped selenium-arsenic alloy comprising less than about 1 percent arsenic by weight may be evaporated by conventional vacuum coating devices to form the desired thickness.
• the amount of alloy to be employed in the evaporation boats of the vacuum coater will depend on the specific coater configuration and other process variables to achieve the desired transport layer thickness. Chamber pressure during evaporation may be on the order of about 4 ⁇ 10 -5 torr. Evaporation is normally completed in about 15 to 25 minutes with the molten alloy temperature ranging from about 250° C. to about 325° C.
• the substrate temperature be maintained in the range of from about 50° C. to about 70° C. during deposition of the transport layer. Additional details for the preparation of transport layers are disclosed, for example, in U.S. Pat. No. 4,297,424 to H. Hewitt.
• the layers containing different additives often utilize a pre-soak "hold" temperature to prevent condensation of selenium alloys that are to be deposited subsequent to the deposition of other selenium layers.
• These crucibles are normally maintained at an elevated temperature while the first coating layer is deposited. It has been discovered that this elevated temperature during the pre-soak hold period causes loss of selenium and selenium rich species from the selenium alloy in the "hold" crucibles and aggravates fractionation.
• the temperatures for crucibles containing alloys for subsequently deposited alloy layers are held at a temperature of less than about 130° C. during the pre-soak hold period when underlying selenium containing layers are deposited.
• the crucible for the top or upper layer alloy has in the past been kept at high temperatures, e.g. at about 190° C.-200° C., to prevent condensation of the base layer material onto the crucibles and the pellets or shot. It has now been determined that such high temperatures can allow the early sublimation of selenium and selenium rich species from the source alloys resulting in a higher than desirable top surface tellurium and/or arsenic concentration on the photoreceptor film.
• the top alloy layer contains selenium alloys with tellurium and/or arsenic in a top or upper layer
• the alloy is kept as cool as practical while avoiding evaporants from condensing on the top layer alloy crucibles during evaporation of the underlying layers.
• the temperature of the alloy should be kept in a temperature zone within which the vapor pressure of the selenium rich species is low, such as below about 130° C. It should be noted that while the pre-soak hold temperature should not be independently controlled above 130° C., it is possible that the heat of condensation may drive the top layer crucible temperature to about 160° C. for a short time.
• Steep temperature ramp heating is desirable for every type of selenium alloy evaporation.
• a temperature ramp from 130° C. to about 385° C. in a period of about 15 minutes is satisfactory.
• the final temperature may range from about 300° C. to about 450° C. for Se-Te alloys.
• tellurium and arsenic fractionation are controlled within narrower limits.
• photoreceptor fabrication yields are improved.
• fractionation is reduced.
• high temperatures are used only during the period of evaporation.
• Photoreceptors having good electrical properties may be fabricated with charge generator layers of selenium alloys having for example, thicknesses of between about 0.5 micrometer and about 10 micrometers.
• Top surface arsenic concentrations can be reduced from several percent to below 1 percent with the materials of this process for SeAs alloys with nominal starting compositions comprising less than 0.5 percent by weight of arsenic.
• Top surface tellurium concentrations can be reduced from in excess of 16 percent by weight to less than about 12 percent by weight with the materials of this process for SeTe alloys with nominal starting compositions comprising 10 percent or less by weight tellurium.
• One photoreceptor preparation control run was made (crucible program A referenced below). Additional runs were made for comparative purposes. Each run was conducted from a batch of about 37.3 kg of selenium-tellurium-arsenic beads formed by water quenching droplets of a molten alloy comprising about 89 percent by weight selenium, about 11 percent by weight tellurium, based on the total weight of the beads, and having an average particle size of about 2200 micrometers. The batch was rapidly ground into a fine powder having an average particles size of about 30 micrometers in a hammer mill grinder (Paudel Grinder, Model 2A, available from Fuji Industries, Japan) for about 5 minutes.
• the ground alloy powder was then compressed into pellets having an average weight of about 300 mg in a pelletizer (Hata Pelletizer, Model HPT-22A, available from Hata Iron Works, Japan). Compression pressure in the pelletizer was about 15000 kg/cm 2 and the pellets had a length of about 3 mm and a diameter of about 6 mm. The resulting batch of alloy pellets was thereafter employed to fabricate a plurality of control electrophotographic imaging members.
• a pelletizer Heata Pelletizer, Model HPT-22A, available from Hata Iron Works, Japan. Compression pressure in the pelletizer was about 15000 kg/cm 2 and the pellets had a length of about 3 mm and a diameter of about 6 mm.
• the resulting batch of alloy pellets was thereafter employed to fabricate a plurality of control electrophotographic imaging members.
• the electrophotographic imaging members were prepared by vacuum evaporating chlorine doped arsenic selenium charge transport alloy material comprising about 99.5 percent by weight selenium, 0.5 percent As and about 20 ppm chlorine, based on the total weight of the layer onto aluminum substrates and thereafter vacuum depositing the selenium-tellurium alloy pellets.
• the chlorine doped arsenic selenium alloy was evaporated from stainless steel crucibles at an evaporation temperature of between about 280° C. and about 330° C. and an evaporation pressure between about 4 ⁇ 10 -4 torr and 2 ⁇ 10 -5 torr.
• This transport layer coated substrate was thereafter coated with the selenium-tellurium alloy pellets described above to form a charge generating photoconductive layer having a thickness of about 5 micrometers and containing about 11 percent by weight tellurium and the remainder selenium.
• This alloy was evaporated at a temperature of between about 300° C. and about 350° C. for stainless steel crucibles at a pressure of about 2 ⁇ 10 -5 torr.
• the resulting electrophotographic imaging members were tested.
• a variety of top layer crucible programs were utilized as identified below (crucible programs B through E referenced below). Quoted times represent the elapsed time for the top layer crucibles at the temperatures specified.
• Top layer crucibles were kept at 21° C. during base layer evaporation. Top layer crucibles were then ramped from 21° C. to evaporation temperature of 335° C. in 12 minutes.
• the improved top layer fractionation characteristics for photoconductors coated under conditions where presoak temperatures are maintained low are shown in Table VI.
• the column on the right displays the 8kV TST values in percent by weight Te from electron microprobe analysis. At 8kV probe energy, the average excitation depth of the detected X-rays is on the order of 0.1 micrometer.
• Conditions C and D indicate lower levels of TST and therefore lower levels of fractionation for those crucible programs where the temperature of the top layer alloy, and therefore the loss of selenium rich species, is maintained low.
• An alloy material comprising about 88 percent by weight selenium and about 12 percent by weight tellurium, based on the total weight of the alloy was initially prepared as beads formed by water quenching droplets of a molten alloy having an average particle size of about 2200 micrometers.
• a first batch of material was abraded and subsequently precrystallized so that the material was 100 percent crystalline as determined by X-ray diffraction.
• This first batch of crystallized alloy was used to fabricate a plurality of electrophotographic imaging members.
• a second batch of amorphous selenium alloy particles were employed to fabricated a plurality of control electrophotographic imaging members. All these electrophotographic imaging members were prepared by evaporating the alloy shot onto aluminum substrates.
• both the amorphous and crystallized alloy shot were held at 200° C. for about 8 minutes. These alloy batches were thereafter evaporated at a temperature of between about 350° C. and about 400° C. from stainless steel crucibles at a pressure of about 2 ⁇ 10.sup. -5 torr. The substrate temperature was maintained at about 60° C. during the evaporation coating operation. The photoreceptors from the batches were tested for top surface concentration of tellurium (TST). Testing was effected by detaching the deposited film from the substrate and determining the tellurium concentration at the top surface by electron microprobe analysis.
• TST top surface concentration of tellurium
• the resulting selenium-tellurium alloy layers had the characteristics listed in Table VII below for samples #1 and #2. Values are listed in percent by weight Te.
• a third set of photoreceptors was coated from the first (precrystallised) batch of alloy in a manner identical with that for samples #1 and #2 except that the temperature of the alloy was raised rapidly to the evaporation temperature of between about 350° C. and about 400° C. without an intermediate hold temperature period at 200° C.
• the resulting selenium-tellurium layers had the characteristics listed in Table VII below for sample #3.

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• 1988
  • 1988-04-08 US US07/179,374 patent/US4842973A/en not_active Expired - Fee Related
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